How do plants sense light and gravity, and how do we use plant hormones in farming?
Auxins as plant hormones controlling growth, phototropism and gravitropism (geotropism), how auxin distribution produces tropic responses, and the commercial uses of plant hormones.
A focused answer to the OCR Gateway GCSE Biology A topic B3 on plant hormones, covering auxins, phototropism and gravitropism, how auxin distribution produces tropic responses, and the commercial uses of plant hormones.
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What this dot point is asking
OCR wants you to describe auxins as plant hormones, explain phototropism and gravitropism in terms of auxin distribution, and state the commercial uses of plant hormones.
Auxins and tropisms
Auxin works by changing how much plant cells elongate (get longer). Where there is more auxin, the cells respond differently in shoots and roots, and this uneven growth makes the plant bend.
Phototropism (response to light)
Shoots are positively phototropic: they grow towards light. This happens because:
- Auxin is made in the tip of the shoot.
- When light shines from one side, auxin moves to (builds up on) the shaded side.
- In shoots, auxin makes cells elongate more, so the cells on the shaded side grow longer than those on the lit side.
- The uneven growth makes the shoot bend towards the light.
This is useful because the leaves receive more light, so the plant can photosynthesise more.
Gravitropism (response to gravity)
Roots are positively gravitropic: they grow towards gravity (downwards), which anchors the plant and finds water. Shoots are negatively gravitropic: they grow away from gravity (upwards), towards the light.
In a root laid on its side, auxin gathers on the lower side. In roots, however, a high concentration of auxin slows cell elongation (the opposite of its effect in shoots). So the upper side of the root elongates more than the lower side, and the root bends downwards.
Commercial uses of plant hormones
Plant hormones (often synthetic versions) are used widely in farming and horticulture:
- Rooting powder. Contains auxins, used to make stem cuttings grow roots quickly so new plants can be grown.
- Selective weedkillers. Contain auxins that make broad-leaved weeds grow too fast and uncontrollably, so they die, while narrow-leaved crops (grasses and cereals) are less affected.
- Controlling fruit ripening. Hormones can delay ripening so fruit can be transported unripe and ripened on arrival, or speed up ripening for sale.
- Producing seedless fruit. Hormones can make fruit develop without fertilisation, giving seedless varieties.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20196 marksA plant shoot is lit from one side and grows towards the light. Explain how auxin produces this response, and explain why this response is useful to the plant.Show worked answer →
A 6-mark extended response on phototropism. Mark it for the auxin mechanism plus the benefit.
Mechanism: auxin is a plant hormone made in the tip of the shoot. When light shines from one side, auxin moves to (accumulates on) the shaded side of the shoot. Auxin causes cell elongation (the cells get longer). Because there is more auxin on the shaded side, the cells there elongate more than the cells on the lit side. This makes the shoot bend towards the light.
Benefit: growing towards the light means the leaves receive more light, so the plant can photosynthesise more and make more glucose for growth. Markers reward: auxin made in the tip; auxin moves to the shaded side; cells on the shaded side elongate more; shoot bends towards light; the benefit of more photosynthesis.
OCR 20213 marksDescribe two commercial uses of plant hormones, and state how plant hormones could be used as a weedkiller.Show worked answer →
A 3-mark recall question on uses.
Two uses (any two): rooting powder (auxins) to make cuttings grow roots; controlling or delaying fruit ripening so fruit can be transported; producing seedless fruit; killing weeds (selective weedkillers).
As a weedkiller: selective weedkillers contain auxins (synthetic plant hormones) that make broad-leaved weeds grow too fast and uncontrollably, so they use up their resources and die, while narrow-leaved crops (such as grasses and cereals) are less affected. Markers reward two uses and the idea that the weedkiller makes weeds grow uncontrollably so they die.
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